This invention relates generally to audio signal noise reducing systems and more particularly concerns signal compression and expansion systems.
The use of compansion techniques in noise reduction is commonly known. Channel noise is frequently reduced by compression of the dynamic range of the incoming signal by a 2 to 1 linear decibel ratio. This permits easy passage of an incoming signal of wide dynamic range through a medium of relatively restricted dynamic range. Such media include recording tapes, telephone lines, phonograph records and the like. As a result of compression, the noise floor of the medium is effectively lowered or, conversely, the signal level intensity is increased in relation to the medium noise floor. Thus, the noise level is masked by the signal level. The signal is then expanded by a 1 to 2 ratio restoring the full dynamic range of the original signal and reducing channel noise.
One of the problems with this type of system is that modulation of the noise floor within the transmission medium by the audio signal restricts the clarity or purity of the audio signal. This modulation, when resulting from low frequency tones having fast rise and rapid decay produces accentuated problems in the absence of high frequency components which would mask the modulation noise. This is commonly referred to as "breathing" or "hiss".
It is known that this problem can be reduced by use of split band or multiple band companding systems, but such systems are complex and produce frequency response and phase errors.
It is also known that noise modulation effects can be reduced by the use of preemphasis in the audio signal path which boosts high frequency content during compression and by the use of complimentary deemphasis during expansion which reduces high frequency noise. But most systems do not have sufficient transmission channel headroom to accomplish this. Consequently, a known method of detection preemphasis employs a detection circuit which weighs high frequency tones more than low, thus reducing headroom requirements and overload problems. But this detection preemphasis has been used only in conjunction with relatively low levels of signal path preemphasis to avoid the common problem of high level, high frequency tape saturation experienced with magnetic recording tape.